The departed Seaman A. Knapp might have thought it a good idea, but the memorial arch bearing his name that spans Independence Avenue may never be the same.

Since early December, the covered walkway that connects the Agriculture Department's South Building and its Administration Building has been emblazoned with a colorful banner promoting an ice-cream exhibit.

While the Knapp Memorial Arch may not rank in the big leagues of national shrines, it nonetheless was accorded a certain sanctity by Congress in 1934. And some denizens of the USDA put its commercialization on a par with desecration.

The banner promotes something called The American Ice Cream Experience, a pictorial trek through the history of frozen delights that is on display in the Administration Building lobby.

"It was promoted by the International Association of Ice Cream Manufacturers," a USDA official said, "and Secretary [John R.] Block liked it. He was right there on opening day, scarfing up the free ice cream. Usually, we charge a rent for things like this, but this one was free."

The association's display pays proper historical obeisance to the likes of Nero and Alexander the Great, early experimenters with iced confections, but actually turns out to be a subtle promotion for the Baskin-Robbins franchise ice-cream outfit.

The exhibit duly records milestones in the entrepreneurial lives of Irv Robbins and Bob Baskin, unabashedly depicted as the men who gave real meaning to ice cream.

Promotion of U.S. dairy farmers' products was right up Seaman Knapp's professional alley. He is credited with the idea that led to creation of the Cooperative Extension Service and its county agent and home economist network early in this century.

The gods may have another view of the placement of banners on shrines, however. A fierce windstorm this week ripped a second ice-cream sign from the top of a second USDA arch over Independence Avenue. That one honors James Wilson, secretary of agriculture under three presidents.